[multipathd] close orphan path file descriptors

[multipathd] close orphan path file descriptors

Maxim Kozover rightly reported :

"multipathd keeps the file /dev/sdx open, regardless if there's a
multipath-related map having that sdx device as a path or not.
It drops /dev/sdx only if the sdx device is removed.
What's bad is that this drop is not always successful and I think it's
not a right way to cope with the situation. Killing multipathd always
releases the descriptors.

Now when I (...) remove maps with multipath -f, the number of
multipathd threads changes accordingly. But it doesn't release
/dev/sdx devices. That means even after I did multipath -f for all
iSCSI devices and multipath -l output is empty, multipathd still has
those /dev/sdx files open and I can't unload iSCSI driver, cause it
has a non-zero usage count."